The present invention is directed to a method for manufacturing ohmic contacts having low transfer resistances on semiconductor material that has been doped or doped.
Typically, in the manufacture of planar hetero bipolar transistors, particularly those based on GaAs, difficulties occur in manufacturing the metallization for the contacting of the active, doped zones, of the transistor with adequately low transfer resistance. In manufacturing npn transistors, n-doped regions in the semiconductor substrate are fashioned for the emitter and for the collector. The base is p-doped by a further implantation. The implants are annealed and activated by heating. The annealing occurs at high temperatures, either based on long-term furnace annealing or rapid thermal annealing in a rapid thermal anneal system.
After the annealing, metal contacts are applied on and alloyed into the doped regions of the transistor. In most known manufacturing processes, the implantations are first annealed at high temperatures, then the metallization is applied, and finally these metal contacts are alloyed in at comparatively low temperatures. Such a process is described in the publication by M.F. Chang et al, "GaAs/(GaAl)As Hetero-Junctin Bipolar Transistors using a Self-Aligned substitutional Emitter Process" in IEEE Electron Device Letters EDL-7, 8 through 10 (1986). In the process a hetero bipolar transistor is manufactured whereby the doping for the base occurs by implantation and the emitter region serves as a masking. In this way, a base region self-aligned to the emitter is produced. The base doping is annealed before the appertaining metallization is applied. The base metallization is adjusted with respect to the doped base region and with respect to the emitter region. This adjustment results through the use of a silicon oxide or silicon nitride mask technique.
Another possible process for the annealing of implants and alloying in of the metallization is described in the publication by K. Ishii et al, "High Temperature Stable W.sub.5 Si.sub.3 /In.sub.0.53 Ga.sub.0.47 As Ohmic Contacts to GaAs for Self-Aligned HBTS" in IEDM 86, pages 274 through 277. The annealing of the doping and the alloying in of the metallization occurs in a one step tempering process. The metallization is first applied and then used as a mask for the implantation of the doping. For the contacts into the appertaining fundamental doping of the semiconductor material a high temperature stable metallization is used. This allows the annealing of the implanted doping and the alloying in of the metallization to occur in one tempering step.
Both of the processes are similar in that the annealing of the implants respectively occurs for regions that are not yet provided with metallizations at the moment of annealing.